A spark plug used as an ignition means in internal-combustion engines (hereafter, simplified to the engines) such as an engine of an automobile, there is a spark plug provided with a spark discharge gap formed by a center electrode and an earth electrode facing in an axial direction.
The spark plug makes the spark discharge gap generates an electric discharge, and ignites a fuel-air mixture in a combustion chamber by the electric discharge.
Here, a gas flow of fuel-air mixture, such as a swirl flow or a tumble flow, is formed in the combustion chamber, and ignition performance can be secured when the gas flow flows moderately at the spark discharge gap.
However, a part of the earth electrodes joined to a tip part of a housing may be disposed at an upstream side of the spark discharge gap in the gas flow depending on the mounting position of the spark plug to the engine.
In this case, the gas flow in the combustion chamber is interrupted by the earth electrode, and there is a possibility that the gas flow near the spark discharge gap may stagnate.
As a result, there is a possibility that the ignition performance of the spark plug may fall.
That is, there is a possibility that a problem may arise that the ignition performance of the spark plug varies depending on the mounting position to the engine.
Although many engines employ lean combustion especially in recent years, there is a possibility that combustion stability may fall depending on the mounting position of the spark plug in such an engine.
Moreover, it is difficult to control the mounting position of the spark plug to the engine, i.e., the position of the earth electrode in a circumferential direction.
This is because the mounting position changes with a form of a screw for mounting in the housing, a degree of tightening the spark plug to the engine at the time of mounting, etc.
Then, in order to suppress blocking of the gas flow by the earth electrode, a composition with a process of opening a hole in the earth electrode, or a composition of joining an earth electrode to a housing by a plurality of thin tabular members are disclosed (refer to Japanese Patent Application Laid-Open Publication No. 9-148045).
Moreover, in order to stabilize a direction of the tumble flow in a combustion chamber, a composition that provides an inclined circumference surface section in a tip part of a housing is also disclosed (refer to Japanese Patent Application Laid-Open Publication No. 2008-108479).
However, with the earth electrode having a hole un the earth electrode as disclosed in '045, there is a possibility of causing strength reduction of the earth electrode.
Moreover, if the earth electrode is thickly formed in order to prevent such strength reduction, it becomes easy to block the gas flow of fuel-air mixture after all.
Similarly, with the inclined circumference surface section in a tip part of a housing as disclosed in '479, manufacturing steps increases due to the shape of the earth electrode being complicated, and the manufacturing cost becomes high.
Moreover, the problem that the earth electrode disposed at the upstream side of the gas flow relative to the spark discharge gap blocks the gas flow is unsolvable with the composition in '479, because it does not specify the formation position of the inclined circumference surface section.